1. Field of the Invention
The invention relates to a doctor blade for machines for paper production and relates in particular to the coating of doctor blades to improve the doctor blade surface.
2. Description of the Related Art
As a rule, the fibrous web, the so-called base-paper web, produced in the paper manufacturing process does not yet possess the surface properties required for specific paper qualities which are to be produced and must therefore be processed accordingly.
For smoothing of the web surface a pasty coating layer consisting of pigments, binding agents and additives is generally applied to said surface. Coating of the paper web can occur in a separate operation, but is however normally integrated into the paper manufacturing process through integration of a coater into the paper machine. Smooth paper surfaces are achieved with the blade coating method, whereby an excess of coating medium is initially applied onto the paper and is then doctored off with a doctor blade. Due to the pressure exerted on the coating medium by the doctor blade which is referred to as a coating blade the indentations on the paper surface are filled with coating medium, thereby achieving a uniform surface on the coated paper.
Surface structures, for example on hygienic papers, are produced according to the creping process, whereby the web is directed over a doctor, which is referred to as creping doctor blade.
Based on the high pressure with which the doctor blade rests on the paper web, whereby as a rule the contact pressure is 150 N/m (contact pressure relative to the length of the doctor blade) or higher, high demands are put upon the wear resistance of the doctor blade. Doctor blades used in surface treatment of the paper web are therefore often manufactured from high-tensile steel.
It is known to provide the surface of doctor blades in the regions in which the blade rests on or respectively makes contact with the coating medium with a coating which has a higher wear resistance than the base material of the doctor blade. Such coatings are generally produced from an abrasion-resistant material by using metal oxides or hard metals into which a metal-carbide is embedded into a cobalt-, nickel- or iron matrix. In order to apply coatings which are physically as well as chemically as homogeneous as possible, thermal spray techniques are preferably used, whereby the coating material is applied in several passes. Each of the passes provides a thin layer of coating material onto the doctor blade, or respectively onto the already previously applied coating layers. Application of the coating in several thin layers ensures that the components of the coating material cannot separate during buildup of the coating. Based on the chemical as well as physical identity of the individual layers a homogenous coating is hereby produced.
In order to prevent developing of doctoring streaks in the coating on the paper web or respectively on the creped paper web the surface of the doctor blade coating is ground smooth after application to an extent that it exhibits roughness values which are as low as possible. With certain combinations of coating material and for materials used in the design of the doctor blade base body it may be necessary to arrange an additional coat between the coating and doctor blade base body which offers better adhesion of the coating on the doctor blade base body.
A further development of an abrasion-resistant doctor blade coating provided with an adhesion layer is introduced in the international patent application WO 2006/134209 A1. The coating features a multi-layer design in which the surface of the coating intended to make contact with the paper web is formed by a hard material, for example tungsten carbide, chromium carbide or titanium carbide, whereas the side of the coating abutting the doctor blade base body is formed by a material with good adhesion properties on the doctor base body. Materials suitable for this layer are titanium oxide and aluminum oxide. The layers arranged between these two outer layers feature a mixture of both layer materials, whereby the portion of one layer material decreases with increasing distance from the coating surface built up from this material.
In the doctoring process the doctor blade exhibits higher wear at the edges of the paper web, than in between, since the outer ends of a doctor blade are generally not in contact with a coating medium and are therefore abraded sooner by the rough surface of the fibrous mat forming the raw paper web than the part of the blade which comes into contact with the coating medium. In order to counter this effect it is suggested in the international patent application WO 98/28089 to provide the doctor blade with an additional layer which is arranged between the doctor blade base body and the actual coating. This intermediate hard material layer possesses a lower wear rate than the useful work coating and can therefore maintain the blade profile over a longer period of time.
If the doctor blade is worn it must be replaced. After replacement it takes however some time until the new blade is “run in” which indicates the fact that, even though the doctor blade coating was ground to precisely match the geometry of the paper web orientation, smaller deviations still exist in practice in both geometries. The doctor blade consequently does not produce the demanded quality of paper web surface. The time to “run in” the doctor blade is the time which is required until the paper surface exhibits the demanded quality and can be as long as an hour.
The paper produced during the “run in” time is of inferior quality and has to be disposed of under certain circumstances.
It is therefore desirable to cite, and what is needed in the art is, a doctor blade for surface treatment of fibrous webs, for example paper webs, which makes shorter run-in times possible.